This is because the ParseInitializeBehaviour is required to be instantiated in the Unity SDK to setup the async-task processing (otherwise async Parse calls never complete) but it doesn't allow you to pass a custom server URL for it to use in the ParseClient.Initialize() call it makes.
Fix: I've created a fork and added a 1.7.1 release with (just) a fix for this issue here.
Note: This issue also seems to be fixed in the latest code in the master branch but I wasn't sure how to use that against my pre-existing project.